(A) Field of the Invention
The present invention relates to a structure and manufacturing method of an over-current protection device. More specifically, the present invention relates to a resettable chip-like over-current protection device utilizing a polymeric positive temperature coefficient (PPTC) material as a substrate thereof.
(B) Description of Related Art
PPTC devices have been widely used in circuits of electronic devices today. The conductive composite material used in the PPTC devices is mostly composed of polyethylene and electrically conductive particles (mostly carbon black). Under normal operating temperatures, polyethylene confines the conductive particles tightly in a crystalline structure thereby to form a low resistance conductive network. When an abnormally high current is present, the heat generated on the device will reform the polyethylene from crystalline to amorphous. In such a situation, confined conductive particles will be separated due to quick expansion of the polyethylene, which breaks original conductive network. As a result, the resistance rises quickly so that the abnormal current passing through the device will be limited. After termination of the abnormal current, the temperature of the device will drop to room temperature and the conductive composite material will return to the original structure, which means that the polyethylene again confines the conductive particles in the crystalline structure, forming a low resistance conductive network, whereby the purpose of automatic resetting is obtained.
Currently, PPTC devices are mostly used for the purpose of over-current protection. In additional to radial-leaded type devices similar to conventional fuses, the PPTC devices are applied to surface-mount type devices used in a printed circuit board (PCB), which is composed of an at least 5-layer structure of a PPTC substrate, two main electrode conductive metal foil on top and bottom surfaces of the substrate, and two surface connecting electrode layers. For instance, U.S. Pat. No. 6,292,088 (entitled “PTC Electrical Devices for Installation on Printed Circuit Boards”) shown in FIG. 1A˜FIG. 1F discloses that on a PPTC substrate 10, two main electrode conductive metal foils 11a and 11b are applied on two surfaces of the PPTC substrate firstly, as shown in FIG. 1A and FIG. 1B so as to form a sandwich structure as shown in FIG. 1C and FIG. 1D, then a via 12 necessary for connecting top and bottom electrode conductive layers is formed, as shown in FIG. 1E and FIG. 1F. Subsequently, a connecting electrode layer 13 is formed on the via 12, as shown in FIG. 1G and FIG. 1H. Then electrode isolation areas 14 required in installation of terminal electrodes of the chip-like resettable over-current protection device are formed, as shown in FIG. 1I and FIG. 1J. Finally, a finished substrate is separated into individual devices according to predetermined cutting lines 15 as shown in FIG. 1K and FIG. 1L, and then the chip-like resettable over-current protection device with two terminal electrodes 16 and 17 is finished. The said terminal electrodes 16 and 17 are isolated from each other but connected to themselves located on top and bottom surfaces.
After analyzing this prior art, it is understood that the prior art has the following drawbacks:                1. The structure of 5-layer “surface connecting electrode conductive layer-main electrode conductive metal foil-PPTC substrate-main electrode conductive metal foil-surface connecting electrode conductive layer” and the manufacturing method thereof are too complex.        2. In preparing the electrode isolation areas, parts of electrode conductive metal foils 11a and 11b on device need to be removed and this process consumes much power and produces pollution.        